Your best bet is to measure you existing door (height and width) and taking those into you local Home Depot and discussing you project with someone in the millwork department. It wouldn't hurt to take a few pictures too.

The strongest door that can be cut to size would be a solid wood door. Steel and fiberglass doors would have to be custom made to size. Keep in mind that even a wood door will have limits on how much can be cut off if it and still be functional.